mac80211: Fix null pointer dereference with iTXQ support

This change adds null pointer check before dereferencing pointer dev on
netif_tx_start_all_queues() when an interface is added.
With iTXQ support, netif_tx_start_all_queues() is always called while
an interface is added. however, the netdev queues are not associated
and dev is null when the interface is either NL80211_IFTYPE_P2P_DEVICE
or NL80211_IFTYPE_NAN.

Signed-off-by: Chunho Lee <ch.lee@newracom.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>